This relates to printed circuit boards (PCBs) for use in high performance circuits. More particularly, it relates to improvements in the interface between the PCB and a semiconductor integrated circuit package mounted on the PCB. A particular application of the invention is with high frequency transceivers.
Since their development in the 1940's, PCBs have been widely used for connecting all sorts of electrical and electronic packages. In their simplest form, PCBs comprise a layer of metal foil in which circuit paths have been defined that is laminated to an insulating substrate. More complex, multi-layer PCBs are formed by providing metal layers on both sides of the insulating substrate to form double-sided PCBs and by laminating together several single-sided and/or double-sided PCBs to form PCBs with many conducting layers. Typically, the metal used is copper; and the insulating substrate is resin-impregnated fiberglass. One widely used laminate is single-sided or double sided FR-4. Further information about PCBs is available in J. A. Scarlett (ed.), The Multilayer Printed Circuit Board Handbook (Electrochemical Publications Limited, 1985) and R. R. Tummala (ed.), Fundamentals of Microsystems Packaging, ch. 16 (McGraw-Hill, 2001), which are incorporated herein by reference.
In recent years, the frequencies of signals transmitted on some of the interconnection paths of a PCB have risen into the GigaHertz (GHz) range. At these frequencies, it becomes desirable to shield the interconnection path; and shielding structures used for many years in conventional microwave technology have been adapted for use in PCBs. FIGS. 1A and 1B depict a top view and a cross-section along line B-B in FIG. 1A of a first conventional PCB 10. In the top view, first and second signal traces 20, 22 run along a top surface 12 of PCB 10 connecting to signal contact pads 30, 32, respectively. Traces 20, 22 run between grounded contact pads 40. Note that the width of the traces is narrowed at 24 to permit them to run between the ground pads. Typically, signal contact pads 30, 32 are located near an outer edge of PCB 10 and the narrow portion 24 of traces 20, 22 is relatively short. As shown in the cross-section of FIG. 1B, PCB 10 is formed in a micro-strip configuration with several ground planes 50 that provide a current return path. The ground planes underlie the signal traces and are separated from each other by insulating layers 60. The ground planes are continuous two-dimensional metal sheets with no openings that are connected together by columns of through vias 70 that extend from contact pads 40 on top surface 12 of the PCB to a bottom surface 14 of the PCB.
Illustratively, PCB 10 is electrically and mechanically connected to a semiconductor integrated circuit package 80 by solder balls or solder bumps 90, 92, 94 on contact pads 30, 32, 40, respectively.
FIGS. 2A and 2B depict a bottom view and a cross-section along line B-B in FIG. 2A of a second conventional PCB 210. In the bottom view, first and second signal traces 220, 222 run along a bottom surface 214 of PCB 210 connecting to signal contact pads 234, 236, respectively. As shown in the cross-section of FIG. 2B, PCB 210 is formed in a micro-strip configuration with several ground planes 250 that are separated from each other by insulating layers 260. The signal contact pads 234, 236 are connected to contact pads 230, 232 on top surface 212 of the PCB by through vias 272, 274 that are electrically isolated from the ground planes by gaps in the planes. Except for the gaps in the ground planes that isolate them from through vias 272, 274, the ground planes are continuous two-dimensional metal sheets with no openings that are connected together by columns of through vias 270 that extend from contact pads 240 on a top surface 212 of the PCB to bottom surface 214 of the PCB.
Signal traces 220, 222 run between vias 270 on bottom surface 214. The width of the traces is narrowed at 224 to permit them to run between the vias. Typically, signal contact pads 234, 236 are located more toward the interior of the PCB and the narrow portion of traces 220, 222 is much longer than the narrow portion of typical signal traces 20, 22 on top surface 212.
Illustratively, PCB 210 is electrically and mechanically connected to a semiconductor integrated circuit package by solder balls or solder bumps 290. 292, 294 on contact pads 230, 232, 240, respectively, on top surface 212 of PCB 210.